Surface treating process for metal parts



United States Patent 921,321 US. 01. 204-32 7 Claims Int. Cl. C23b /00; czsb 3/00,- B24c 1/08 This invention relates to the surface treatment of metallic parts, and its main object is to provide a method of surface treatment whereby a metal part can be maintained in frictional and or stationary contact engagement with another part without lubrication between the engaging surfaces, for prolonged periods of time and without any danger of binding or seizure between the surfaces.

There are many circumstances in which it is extremely desirable or indispensable to operate mechanical apparatus containing relatively moving parts without lubrication, and under strenuous conditions. Thus in nuclear plants, machinery must operate at relatively high temperatures, e.g. to 600 C., and under exposure to neutron or the like radiations which would rapidly cause breakdown of any lubricant compositions that may be provided. Moreover such machinery is difficult of access for maintenance and repairs owing to the radioactive surroundings in which it must operate.

It has previously been attempted to impart to a metallic surface an extremely fine or mirror-smooth surface condition, and/or to provide a smooth metallic coating thereover as by electroplating, in an endeavour to render the surface suitable for prolonged frictional contact engagement with another surface without danger of binding. Present-day techniques make it possible to comparative ly easily obtain surface conditions of extreme smoothness in which the residual asperities and cavities are only a small fraction of one micron in average dimensions by such means as machine grinding and buffing. It was believed that a metallic surface having such smoothness should be capable of prolonged frictional contact without lubrication. Contrary to expectations however, experience has shown that this was not in fact the case, and previous attempts of this kind have failed.

The applicants investigations have now shown that the relatively early and heretofore unexplained break down of an apparently mirror-smooth metallic surface in frictional contact with a similar surface under lubricantfree conditions is due to the presence just beneath the polished metallic surface, of an irregular, heterogeneous layer, which results from the strains and deformations which the metal structure has sustained during the grinding and/or bufling operations. This heterogeneous layer impairs the cohesion of the metal and leads to a prompt breakdown of the overlying surface when subjected to frictional stresses.

The present invention is based on that discovery. In accordance with a chief aspect hereof, a method of producing a metallic surface capable of withstanding prolonged lubricant-free frictional contact with another surface, comprises the steps of polishing the surface of the metal to a mirror-smooth condition by conventional techniques, removing a surface layer of the metal to a determined depth such as to eliminate the afore-mentioned heterogeneous layer resulting from the polishing operations, and then depositing a metal coating.

The initial step of the process may comprise a series of conventional grinding and buflin-g operations and should be sufficiently thorough to reduce the surface asperities and other irregularities to dimensions less than about 10 to 16 microinches, ie. 0.00025 to 0.00040 mm. as tested by means of a diamond-tip roughness-meter for example. Such a degree of smoothness can be obtained by means of any of various types of rectifying machines currently available, and is possibly supplemented by a series of buffing steps, preferably in the presence of water.

The next step of the process of the invention is to remove a surface layer of the polished metal, preferably by sand-blasting, the abrasive particles being projected in a stream of pure water at high velocity.

The third step may comprise coating, preferably electrocoating with a suitable metal.

It has been found that a metal part when prepared by the process of the invention is able to withstand frictional contact with another metal or plastic part of comparable surface characteristics, for prolonged periods of time without any lubricant and under strenuous operating conditions, without breakdown. As indicated above this remarkable result is to be attributed to the removal, according to the invention, of the microscopic heterogeneous sublayer of the metal produced as a result of the grinding and/ or bufiin-g operations.

When the metal part has been subjected to such polishing. operations in accordance with the initial step of the process of the invention as described above, microscopic examination of a section of the part shows the presence of an extremely thin layer, a few microns in depth, wherein the metal crystals are broken down and distorted apparently due to the grinding to which the metal was subjected. In this layer the metal grains are seen to be largely isolated from one another and separated by gaseous inclusions, mainly residual nitrogen from the combustion of the air caused by the temperature elevation during the grinding step, so that the cohesion of the metal is greatly diminished. Moreover various chemical compounds are found to be present in large numbers at the inter-grain boundaries chiefly including oxides formed at the aforementioned elevated temperatures due to the presence of water, air, coolant and lubricant fluid used during the machining operations, and other ingredients. These compounds present partly in the solid and partly in liquid and vapour phases, in addition to reducing inter-grain coehesion, tend to expand and create inter-grain tension forces. A layer of this character is evidently physically unstable.

When a metallic surface including an unstable layer of this kind is contacted with another, say similar, surface, the unstable layer will rapidly exert destructive effects on the assembly, due to a number of causes.

In the first place, it is clear that when a pair of contacting surfaces are considered, the common contact surface area decreases as the r-oughness increases, and local over-pressures are created. Due to such over-pressures, metal particles are sheared and stripped off, all the more readily because of the existence of the inter-grain tensions mentioned above. The stripped-off particles, in the absence of any lubricant to carry them away, will remain in place and will oxidize, increasing in volume and adding to the oxides already present at the inter-grain boundaries, thus amplifying the tension forces and leading to further over-pressures and the strip-ping off of more particles. Since such metalstripping actions generate heat, the temperature rises and the above cumulative effects are thereby further amplified.

In the case where the metal surface under consideration remains in static contact with its mating surface under the lubricant-free condition considered, inter-grain corrosion initiated by the mechanism just outlined will propagate deep into the mass of the metal, and may eventually lead to breakage of the metal part. Where on the other hand the metal surface is in relative motion with respect to its mating surface, in the absence of lubricant, the asperities created at the surface will act as tools cuting into the mating surface and both surfaces are rapidly put out of commission, and will bind or seize. All such effects are of course magnified if adverse surrounding conditions are present, such as vibrations, temperature elevation, temperature variations, and corrosive atmosphere.

The above destructive effects are all eliminated by the second step of the process of the invention which comprises the removal of the heterogeneous layer produced by grinding. As indicated above fine sand-blasting is preferably used for this purpose, using abrasive sand particles in an approximate range of sizes of from 2 to 100 microns, preferably with a major proportion of particles about microns in size. The sand blast is projected onto the metal surface in a stream of pure water at high velocity, e.g. about 300 meters per second, normally to the surface. The abrasive particles when applied in this manner possess sufficient kinetic energy to remove any broken-down and loosely-bonded grains of metal and compounds in the unstable heterogeneous layer. The outlined blasting procedure avoids unwanted temperature elevation.

According to the invention this blasting step should be conducted under conditions of strict metallic asepsis, i.e. so as to avoid introducing metallic inclusions into the treated surface comprising metals differing in nature from the constituent metal of the surface undergoing treatment. The presence of such foreign metal inclusions would be liable to cause oxidations through galvanic effects. The desired metallic asepsis is obtained by performing the sand-blasting treatment in an enclosure and With all the components of the sand-blasting apparatus used, including said enclosure, the container for abrasive material, and the sand-blasing conduits and nozzles, all being made from suitable plastic materials.

The sand-blasting operation is continued for a time that should be determined in each case by test, sufficient to ensure the full removal of the heterogeneous layer.

The healthy, homogeneous metal surface thus obtained is then washed with distilled water, and dried in filtered air, and metal-coated in the next step of the process of the invention. The metal coating may be deposited by any suitable means, electrolytic, chemical, or electrochemical in character. Electrolysis in a solid medium, as presently described, is preferred because it avoids the risk of introducing hydrogen inclusions as might be the case if an ordinary electrolysis bath were used. Such hydrogen inclusions would impart fragility to the metal coating. During the aforementioned washing, drying and electroplating steps the part is manipulated under conditions of strict cleanliness.

In the solid-medium electroplating process referred to, the part being treated is preferably connected to negative polarity so as to constitute the cathode. A graphite anode is used, cased in a cotton envelope impregnated with the electroplating solution used. The assembly comprising the graphite anode and its cotton envelope is continuously reciprocated past the part being treated so as to ensure a uniform deposition throughout the surface to be coated.

According to the invention, the coating metal applied to the prepared surface of the part is selected to have a very low friction coefficient. In a preferred form of the invention, the metal coating comprises two or more coatings of different metals, successively deposited. Thus, an initial or base layer of one metal may be electrodeposited by the procedure described above, and the resulting base coating subjected to a mild buffing action limited to the removal of any sharp points that may have been formed by electrical point effects liable to arise during electrolysis. The bufiing may be performed by means of a fine-grained cloth lubricated with pure water. A

final metal coating may then be deposited using a similar electrolytic procedure.

The applicants experiments have shown that minimum friction and best all-round results are obtained when the metal parts to be held in static or dynamic frictional engagement with each other in the absence of lubricant, have different coating metals at the contacting surface. Thus, excellent results have been obtained when one of the parts had an outer coating of chromium about 3 microns deep, and the mating part an outer coating of silver, about 10 microns deep. In this instance the first part was provided with an underlying base layer of nickel, about 10 microns deep, and the other with a base layer of gold about 2 microns deep.

The metals used in the coating operations should ing with the underlying metal as a result of the electromoreover be selected so as to be capable of strong bonddeposition process.

The metal coated part is then preferably exposed to a diffusing heat treatment. The treatment involves heating the part to within the diffusion range of the coating metal used, e.g. about 250 C. Where the outer coating is gold or silver, in a non-oxidizing atmosphere. This treatment brings about a diffusion of the coating metal between the grains and enhances the final surface condition.

It will be understood that both during and after the operations described, the parts should be handled, and stored, with extreme care so as to avoid damaging the surfaces both mechanically and physicochemically as through attack or oxidation. Such handling, as well as any assembly operations involving the finally treated parts, should be performed in conditions of perfect cleanliness and in a dust-free atmosphere so as to prevent potentially abrasive and/or corrosive particles from settling on the surfaces.

The metal surface treatment of the present invention is applicable to a wide variety of mechanical components designed for prolonged lubricant-free operation in service. As one example, the invention was applied to the components of a pressure-gas ram actuator forming part of machinery in a nuclear plant which was to operate at elevated temperature in radioactive surroundings without possibility of lubrication. The engaging surfaces of the relatively movable parts of the actuator assembly in contact with plastic seal-rings, were treated in accordance with the invention, using chrome as the outer coating. The mating surfaces of guide rings engaging the chrome-plated actuator parts, were similarly treated according to the invention, using silver as the outer coating metal.

As another example the invention was applied to a push-rod used to transmit linear displacements, slidable within a bore and cooperative with annular sliding seal members. The surface of the rod Was treated according to the invention, with chromium as the outer coating metal. The other mating bore surface was similarly treated with silver as the outer coating metal. Instead of chromium and silver, silver and gold may be used as the outer coating metals respectively.

In all cases it has been found that mechanical parts treated in accordance with the invention are capable of lasting operation Without lubricant. Thus the push-rod last mentioned was subjected to 100,000 reciprocations without lubrication at a temperature of about 250 C., without binding occurring. A similar rod surface-treated by conventional methods, and operated under similar conditions was disabled through binding after only a few tens of reciprocations.

It will be understood that variations may be introduced into the procedure described herein without exceeding the scope of the present invention.

What I claim is:

1. A method for surface treating metallic parts designed to allow for an unalterable fitting during static or dynamic contact with other metallic pieces comprising the following phases:

(a) a first phase consisting in polishing the surface of each piece in order to bring it to a state of high polish,

(b) a second phase consisting in applying upon said surface of each piece an abrasive jet of sand carried by a current of water, to eliminate the superificial layer altered during the polishing of said surface,

(c) a third phase consisting in applying by electrolysis upon each surface a metallic coating constituted by at least two superposed layers of different metals with the layers forming the terminal coating of each of said pieces consisting of a different metal.

2. A process as set forth in claim 1 wherein said first phase consists in bringing the surface of each piece to a state of polish whose roughness has a value of about to 16 micro inches.

3. A process as set forth in claim 1 wherein said second phase consists in a sanding of each polished surface by means of grains of sand having a dimension from 2 to 100 microns mixed with a current of pure water projected upon the surface treated at a speed of the order of 300 m./sec.

4. A process as set forth in claim 3 wherein the dimension of the grains of sand is approximately 10 microns.

5. A process as set forth in claim 1 wherein there is established upon each surface electrolytically a lower layer of a predetermined metal, then subjecting such layer to a slight polishing designed solely to erase the summits created by the point phenomena produced in electrolytic operations and finally applying electrolytically upon such polished layer a final metallic layer of a different metal.

6. A process as set forth in claim 1 wherein there is applied successively upon one of said surfaces a layer of nickel and a layer of chromium and upon another surface a layer of gold and a layer of silver.

7. A process as set forth in claim 1 wherein said coating is subjected to a thermic treatment to assure inter granular diffusion of the coating.

References Cited UNITED STATES PATENTS 1,383,175 6/1921 Udy et al 20437 X 1,792,082 2/1931 Fink et a1 20437 X 2,711,389 6/1955 Beach et al 20432 3,202,589 8/1965 Bride 20432 FOREIGN PATENTS 789,513 1/ 1958 Great Britain.

OTHER REFERENCES The Extending Field for Vapor Blasting, Machinery, vol. 84, Jan. 22, 1954, pp. 169-176.

RALPH S. KENDALL, Primary Examiner.

US. Cl. XR. 

1. A METHOD FOR SURFACE TREATING METALLIC PARTS DESIGNED TO ALLOW FOR AN UNALTERABLE FITTING DURING STATIC OR DYNAMIC CONTACT WITH OTHER METALLIC PIECES COMPRISING THE FOLLOWING PHASES: (A) A FIRST PHASE CONSISTING IN POLISHING THE SURFACE OF EACH PIECE IN ORDER TO BRING IT TO A STATE OF HIGH POLISH, (B) A SECOND PHASE CONSISTING IN APPLYING UPON SAID SURFACE OF EACH PIECE AN ABRASIVE JET OF SAND CARRIED BY A CURRENT OF WATER, TO ELIMINATE THE SUPERIFICIAL LAYER ALTERED DURING THE POLISHING OF SAID SURFACE, (C) A THIRD PHASE CONSISTING IN APPLYING BY ELECTROLYSIS UPON EACH SURFACE OF METALLIC COATING CONSTITUTED BY AT LEAST TWO SUPERPOSED LAYERS OF DIFFERENT METALS WITH THE LAYERS FORMING THE TERMINAL COATING OF EACH OF SAID PIECES CONSISTING OF A DIFFERENT METAL. 